The present invention relates to a commutatorless DC motor drive device.
A conventional commutatorless DC motor drive device will be described with reference to FIGS. 1 and 2. Coils 2 and 3 and coils 4 and 5 are arranged coaxially on the surface of a stator yoke 1. The coils 2 and 3 are series-connected to form a stator coil L.sub.A for one phase A, while the coils 4 and 5 are also series-connected to form a stator coil L.sub.B for the other phase B. In correspondence to the stator coils L.sub.A and L.sub.B on the surface of the stator yoke 1, a rotator magnet 6 is magnetized to have ten poles in such a manner that the magnetic flux distribution is sinusoidal, as indicated by a two-dot chain line in the figure. The rotor magnet 6 thus magnetized is rotatably arranged. The stator coils L.sub.A and L.sub.B forming the two phases are so arranged that the phases thereof are different from each other by an odd number times 90.degree. in electrical angle. In correspondence to the stator coils L.sub.A and L.sub.B of the phases A and B, Hall elements 7 and 8 are arranged at positions to detect the poles of the rotor magnet. That is, they are arranged at positions different in phase by an electrical angle of 90.degree..
The drive circuit for this arrangement is constructed as follows: The output of the Hall element 7 corresponding to the phase A is applied to an amplifier 9, the output of which is applied to an output circuit 10. The output of the output circuit 10 is connected to the stator coil L.sub.A. For the other phase B, the Hall element 8, an amplifier 11, an output circuit 12 and the stator coil L.sub.B are connected similarly as in the case of the phase A. The input terminals of the Hall elements 7 and 8 are connected in series and are then connected to a current controller 13. The current controller 13 operates to control the input currents of the Hall elements 7 and 8 according to a signal voltage proportional to the rotational speed of the motor which is obtained through a frequency-to-voltage converter 15 and a low-pass filter 16 from the coil 14 of a sensor adapted to detect the rotational speed of the rotator magnet 6.
Thus, in the conventional drive circuit, the input currents of the Hall elements 7 and 8 are controlled by the output of the frequency generator 14, whereby the output voltages of the Hall elements 7 and 8 are varied to control the amounts of current fed to the stator coils L.sub.A and L.sub.B are thereby to control the rotational speed of the rotor magnet 6.
The operating conditions of the motor will now be considered. In the case where the motor is operated at a low speed or under a light load, the amounts of current fed to the stator coils L.sub.A and L.sub.B may be small. In this case, the output voltages of the Hall elements 7 and 8 are low, and therefore the offset voltages of the amplifiers 9 and 11 cannot be neglected.
In other words, the output voltage V.sub.H of the Hall element as shown in FIG. 3(a) is added to the offset voltage V.sub.O of the amplifier as shown in the FIG. 3(b). The resulting voltage has a waveform in which the peak value on the positive side is different from that on the negative side. Since this voltage is amplified, a current I.sub.L having an amplitude on the positive side different from that on the negative side, as shown in FIG. 3(c), is applied to the stator coil. Thus, it is impossible to rotate the rotor stably.